Electronically resettable fuse

ABSTRACT

An electronically resettable fuse includes a saturable core current sensing transformer having two outside legs and a center leg. An oscillator is connected to balanced oscillator windings wound about the outside legs so as to produce an AC flux which saturates the outside magnetic path at waveform peak points. A DC current whose magnitude is to be sensed is passed through a control winding wound about the center leg, and produces a DC flux in the center leg which completes its loop through the two outside legs, only during intervals when the outside path is unsaturated. This completion occurs twice per cycle and is sensed by a sense winding also wound about the center leg. When the current in the control winding surpasses a bias level, the induced DC flux direction reverses and a 180* phase shift across the sense winding occurs. This phase reversal is sensed by a phase detector and utilized to open relay contacts connected to the control winding. External means for resetting the relay from a remote location are also provided.

United States Patent [72] Inventors [54] ELECTRONICALLY RESETTABLE FUSE8 Claims, 2 Drawing Figs.

[52] US. Cl 317/9, 317/33 [51] Int. Cl i. H02h 3/08, l-lO2h 7/26 [50]Field of Search 323/89 (orig), 89 (XR), 89 (X); 317/27, 33, 43, 48, 9,39,

[56] References Cited UNITED STATES PATENTS 2,585,332 2/1952 Logan317/43 Primary Examiner-J. D. Miller Assistant Exam iner-l-larveyFendelman Attorneys-R. F. Kempf, E. Levy and G. T. McCoy ABSTRACT: Anelectronically resettable fuse includes a saturable core current sensingtransformer having two outside legs and a center leg. An oscillator isconnected to balanced oscillator windings wound about the outside legsso as to produce an AC flux which saturates the outside magnetic path atwaveform peak points. A DC current whose magnitude is to be sensed ispassed through a control winding wound about the center leg, andproduces a DC flux in the center leg which completes its loop throughthe two outside legs, only during intervals when the outside path isunsaturated. This completion occurs twice per cycle and is sensed by asense winding also wound about the center leg. When the current in thecontrol winding surpasses a bias level, the induced DC flux directionreverses and a 180 phase shift across the sense winding occurs. Thisphase reversal is sensed by a phase detector and utilized to open relaycontacts connected to the control winding. External means for resettingthe relay from a remote location are also provided.

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INVENTOR Roland E. Genter Arromvns amass EILECTRONKCALLY RESETTAELE FUSEORIGIN OF THE INVENTION The invention described herein was made in theperformance of work under a NASA contract and is subject to theprovisions of section 305 of the National Aeronautics and Space Act ofi958, Public Law 85-568 (72 Stat. 435; 42 USC. 2457).

BACKGROUND OF TI IE INVENTION Various types of apparatus for fusingelectronic circuits are well known. They vary from simple fuses whichare destroyed when an excessive current condition occurs to complexfuses which drop out and reclose through several cycles beforecompletely locking out when an excessive current condition occurs.

In the past, spacecraft experiments have been reluctant to allow fusesin the power line to their experiments for fear that a small surge incurrent might cause a fuse to blow and permanently kill theirexperiment. Moreover, it is difficult to determine the precise amount ofcurrent necessary to blow a fuse because once the precise amount ofcurrent is determined, the fuse is destroyed and, hence, must bediscarded. While recycle type of fuse systems could be used, they arenot satisfactory because they are overly complex and bulky.Consequentiy, there is need for a fuse that will blow when an excessivecurrent occurs but can be reset from a remote source, such as by a pulsegenerated by a ground station and transmitted to a spacecraft, forexample.

If the fuse is to be utilized on a spacecraft, it will be appreciatedthat it must meet certain other requirements. For example, the fuse musthave a minimum voltage drop, minimum power requirements, small weightand small volume. In addition, it must operate at relatively low ampereratings such as one-sixteenth or one-eighth amp., and incorporate slowblow characteristics with inverse time delay for transient loads.Moreover, the fuse must be operable under fluctuating voltage conditionswithout blowing if the load is driven from a fluctuating voltage source.

Therefore, it is an object of this invention to provide a new andimproved fuse.

it is a further object of this invention to provide an electronic fusethat is resettable.

it is a still further object of this invention to provide anelectronically resettable fuse that can be electronically reset from aremote source.

It is still a further object of this invention to provide anelectronicaliy resettable fuse suitable for use on a spacecraft that hasa minimum voltage drop, power requirement weight and volume, andoperates at a low ampere value over a relatively wide voltage range.

SUMMARY OF THE INVENTION In accordance with the principle of thisinvention an electronically resettable fuse is provided. The fuseincludes a saturable transformer having two outside legs and a centerleg. Balanced oscillator windings are wound around the outside legs andconnected to an oscillator so that the outside path is saturated atwaveform peak points. The center leg includes a first winding connectedso as to sense current flow to a load and a second winding connected toa phase detector. The phase detector receives a second signal from theoscillator. In addition, the second winding is biased to a predeterminedievel. in operation, because theoscillator windings are balanced and inphase, the AC flux developed by these windings cancel in the center leg.When a DC current is passed through the first winding, a DC flux isproduced in the center leg which completes its path through the outsidelegs. Due to the saturation effect of the AC excitation, the DC fluxpath is only completed during periods of unsaturation in the Outsidelegs. This completion occurs at twice the excitation rate. Because thesecond winding is biased, the flux induced by the first winding willexceed the flux induced by the second winding at a predetermined level.When this level is reached, the phase of the AC flux induced in thesecond winding reverses. This phase reversal is sensed by the phasedetector which generates a control signal that is sensed by a relaydriver circuit. The relay driver circuit operates a relay that hascontacts in the power line circuit to the load.

In accordance with a further principle of this invention, theoscillator, phase detector and relay driver circuit are formed ofsolid-state components which have low power consumption. Moreover, therelay is a latching relay that only draws power when it receives acontrol pulse.

In accordance with still further principle of this invention, means areprovided for externally resetting the relay from a contact opencondition to a contact closed condition. Moreover, external overridemeans are provided for changing the relay from its normally contactclosed condition.

It will be appreciated by those skilled in the art and others from theforegoing brief summary that the invention provides an electronicallyresettable fuse that senses the DC current flow from a DC source to aload. When the DC current flow exceeds a predetermined level, a phasedetector senses a phase reversal and generates a signal that causes thecircuit between the DC source and the DC load to open. The inventionoperates when a gradually increasing DC current change occurs or when aninstantaneous current change occurs. Moreover, the invention operates atlow current levels and over widely fluctuating voltage ranges such asfrom 30 to volts, for example. Finally, the invention is suitable foruse on a space vehicle because it can be reset from a remote location,such as a telemetered pulse from a ground station.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing objects and many of theattendant advantages of this invention will become more readilyappreciated as the same becomes better understood by reference to thefollowing detailed description when taken in conjunction with theaccompanying drawings, wherein:

FIG. I is a schematic diagram illustrating a preferred embodiment of theinvention; and

FIG. 2 is a waveform diagram illustrating the waveform of the currentsin the various windings of the transformer illustrated in FIG. 1 forvarious current'conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates a preferredembodiment of the invention and generally comprises: a current sensingtransformer ll; an oscillator IS; a phase detector 15; a relay drivecircuit 17; and, a latching relay l9. The current sensing transformer IIin cludes a saturable core having two outside legs 2i and 23 and acenter leg 25. Balanced oscillator windings A and B are wound about theoutside legs 21 and 23; a sense winding C is wound about the center leg;and, a control winding D is also wound about the center leg 25. Thecontrol winding D is connected in circuit between an input terminal 27and an output terminal 29.

The balanced oscillator windings A and B are connected in series acrossthe output of the oscillator l3. Because the oscillator windings arebalanced and in phase, the AC flux developed by these windings cancelsin the center leg 25. The purpose of the AC excitation is to produce anAC flux which saturates the magnetic path through the outside legs atwaveform peak points. Assuming that a DC current is passing through thecontrol winding D, a DC flux is produced in the center leg 25 whichcompletes its loop through the two outside legs 21 and 23. Due to the ACexcitation previously described, the outside path cyclically saturatesand unsaturates. Because of this cyclic operation, the DC flux path iscompleted only during intervals when the outside path is unsaturated.when the path through the outside leg is saturated it acts essentiallyas an open circuit for the DC flux. Eecause the outside path is movedinto and out of saturation twice per excitation cycle, the DC flux pathis completed twice per excitation cycle. Consequently, the AC voltagethat appears across the sense winding C is twice the frequency of theoscillator output.

lFlG. 2 illustrates idealized waveforms for the current flowing throughthe various windings of the current sensing transformer 11 underdifferent conditions. More specifically the first waveform lineillustrates the waveform of the current flow through windings A and B;the second waveform line illustrates the waveform of the current flowthrough the control winding D; and the third waveform line illustratesthe waveform of the current flow through the sense winding C. Theleftmost portion of FIG. 2 illustrates the waveforms for a positivecurrent flow through control winding D; the center of FIG. 2 illustratesthe waveforms when zero current is flowing through control winding D;and, the rightmost portion illustrates the waveforms when a negativecurrent flows through control winding D. it will be noted from viewingFIG. 2 that the induced DC flux direction reverses when the currentthrough the control winding reverses. This reversal creates a 180 phaseshift in the AC induced in the sense winding. Preferably, the turnsratio between the control and sense windings is rather high, such aslzlOOO, for example. As hereinbefore described in actual use, a DC biascurrent is passed through the sense winding in the direction of the fluxinduced by a positive current flow through the control winding D. Whenbiased in this manner, the current flow through the control winding mustsurpass a predetermined level before phase reversal occurs. As alsohereinafter described, it is this phase reversal that is sensed tocontrol the operation of the latching relay 111.

The oscillator 13 illustrated in FIG. 1 comprises: two PNP transistorsdesignated Q1 and O2; to NPN transistors designated Q3 and 1; a diodedesignated D1; eleven resistors designated R1, R2, 113,124,115, R6, R7,R3, R9, R10 and R11; and, five capacitors designated C1, C2, C3, C4 andCS.

The emitters of Q1 and 02 are connected together and to a voltage sourcedesignated +V. The base of O1 is connected through R1 in series with C2to the collector of Q2 and the base of O2 is connected through R2 inseries with C 1 to collector Q1. The junction between R1 and C2 isconnected through R4 in series with R3 to the junction between R2 andC1. The junction between R3 and R4 is connected to ground.

The collector of O1 is connected through R7 to the collector of Q3 andthe collector of O2 is connected through R3 to the collector of Q4. Thecollector of O1 is also connected through C3 in parallel with R5 to thebase of Q4. The collector of O2 is also connected through C4 in parallelwith R6 to the base of Q3. The emitters of Q3 and Q4 are connectedtogether and to ground. The emitter of Q3 is also connected through R9to the anode of D1. The junction between R9 and D1 is connected to oneend of winding A of the current sensing transformer 11. The collector ofO4 is connected through R10 in series with C5 and R11 to ground. Thejunction between C5 and R11 is connected to one end of winding B ofcurrent sensing transformer 11. The other ends of winding A and B areconnected together.

From the foregoing description, it will be appreciated by those skilledin the art and others that the oscillator 13 is a complementarytransistor oscillator well known in the art. It has particularadvantages for use with this invention because of its low powerrequirement and low output impedance. The output is capacitively coupledthrough C5 to the balanced oscillator windings A and B of the currentsensing transformer 11. in addition, as is hereinafter described, theoutput is capacitively coupled to the phase detector 15. As is wellknown, the complementary transistors of the oscillator 13 alternatelyswitch on and off thereby generating a square wave signal at a frequencydetermined by the specific value of the components used. However, any ACgenerator which has a waveform which results in saturation in the outerlegs of the transformer for a portion of each half of the AC cycle willsatisfy the drive requirements for the transformer.

The phase detector 15 illustrated in FIG. 1 comprises: three NPNtransistors designated Q3, Q6 and 07; two PNP transistors designated Q3and 09; a field effect transistor designated Q11 eleven resistorsdesignated R12, R13, R1 R15, R16, R17, R18, R19, R21), R21 and R22; and,two capacitors designated C6 and C7. The junction between C3 and R11 ofthe oscillator 13 is connected through R12 to the base of Q5. The baseof 03' is also connected through C15 to ground and the emitter of O5 isconnected directly to ground. The collector of O5 is connected throughR13 to +V.

The junction between R13 and the collector of O5 is connected through C7in series with R11 to the collector of Q3 and to the base of Q9. Theemitters of OR and 09 are connected to +V. The base of O9 is connectedthrough R13 to +V and the base of O3 is connected to the collector ofQ6. The collector of O9 is connected through R16 in series with R17 toground.

The base of O6 is connected to one end of winding C of the currentsensing transformer 11 and through R18 to +V. The collector of Q6 isconnected through R19 and +V and, the emitter of O6 is connected to theemitter of 0'7 and through R211 to ground. The collector of O7 isconnected to +V and the base of O7 is connected to the other end ofsense winding C of the current sensing transformer 11. The base of O7 isalso connected to the source of Q10. The gate of Q10 is connected toground. And, the drain of Q10 is connected through R21 in series withR22 to ground.

The phase detector 15 detects when a phase reversal occurs in the signaldetected by the sense winding C. in operation, a differential amplifierconsisting of transistors Q6 and Q7 and their associated resistorsamplifies the signal sensed by sense winding C. The amplified signal isessentially a square wave and is applied to Q8. Q8 either inhibits thesignal or allows a negative spike to pass through C7 to the base of Q9.The negative spike turns on Q9. The phase relationship between thesignal sensed by sense winding C and the signal generated by oscillator13 is such that under normal operating conditions O8 is on when thenegative pulse through C7 occurs, and, thereby prevents Q9 from turningon. However, if the bias current point for the sense winding determinedby the voltage passing from +V through R18, sense winding C, Q11) andR21 and R22 to ground is exceeded the phase of the sense winding signalshifts as hereinbefore described. When the phase of the sense windingsignal shifts 180 O3 is off when the pulse through C7 occurs and, hence,O9 is turned on. When 03 is on and a spike occurs, a pulse is applied tothe relay driver circuit 17 which, is hereinafter described, energizeslatching relay 19.

The relay drive circuit 17 illustrated in F IG. 1 comprises: a PNPtransistor designated Q11; and NPN transistor designated Q12; fourresistors designated R23, R24, R25 and R26; and, two capacitorsdesignated C8 and C9. The junction between R16 and R17 of the phasedetector 15 is connected through R23 and R24 to the base of Q12. Theemitter 01012 is connected to ground. The junction between R23 and R2 1is connected through C3 to ground and to the cathode of D1 of theoscillator 13. The collector of Q12 is connected through R25 in serieswith R26 to +V. The junction between R23 and R24 is also connectedthrough C9 to the collector of 011. The junction between R25 and R26 isconnected to the base of Q11 and the emitter of 011 is connected to +V.

The relay drive circuit has delay characteristics which are controlledby the values of R16, R17 and C8. That is, if the series of pulses fromO9 are present for a sufficient period of time, depending on the valuesof these components, the voltage on C3 will reach the turn on thresholdlevel of Q12. When Q12 turns on, it turns on 011 which as hereinafterdescribed applies a voltage to the set coil of the latching relay 19. ifthe pulses from Q9 are present for an insufficient period of time Q12and, hence, 011 are not turned on. R9 and D1 of the osciliator circuit13 are contained in the overall system to as sure that the relay drivecircuit reacts to a very high current overload which could be sufficientto saturate the magnetic core of the current sensing transformer 11.

The latching relay 19 illustrated in FIG. 1 comprises: a set coildesignated S; a reset coil designated R; and a set of relay contacts.One side of the set coil 5 is connected to ground and the other side isconnected to the collector of 011, hence, Q11 is connected so that itenergizes or drives S. The same side of S that is the collector of Q11is connected to the cathode of a diode designated D2. The anode of D2 isconnected to a set terminal 31 so that an external set signal can beapplied to the set coil S, if desired.

One side of the reset coil R is connected to ground and the other sideis connected to the cathode of a diode designated D3. The anode of D3 isconnected to a reset terminal 33 so that an external reset signal can beapplied to the reset coil.

The contacts of the latching relay 19 include a moveable contact element35 and upper and lower contact points 37 and 39. The moveable contactelement 35 is connected to the output terminal 29 and is moveable so asto come in contact with the upper and lower contact points 37 and 39.The upper contact point 37 is connected through the control winding D ofthe current sensing transformer 11 .to the input terminal 27. The lowercontact point 39 is unconnected. Normally, the moveable contact element35 is in contact with the upper contact point 37 so that current canflow from the input terminal to the output terminal. When a currentsurge occurs, the current sensing transformer ll senses the surge andthe previously described phase reversal occurs. The phase reversal issensed by the phase detector and a pulse is applied to the relay drivercircuit 17, which after a suitable delay, applies a pulse to the resetcoil 8. This latter pulse causes the moveable contact element 35 to moveout of contact with the upper contact point 37 and move into contactwith the lower contact point 39. Hence, the circuit between the inputterminal 27 and the output terminal 29 is broken. Thereafter, when areset pulse is applied to the reset terminal 33 the moveable contactelement reverts to its prior or normal position and the circuit betweenthe input terminal 27 and the output terminal 29 is again completed. Theset terminal 3i allows an external pulse to be utilized to open circuitthe system.

It will be appreciated from the foregoing description that anelectronically resettable fuse is provided by the invention. Regardlessof whether the excessive current occurs slowly or rapidly, the fusecircuit operates to sense when the current from the input terminal tothe output terminal surpasses a predetermined point. When this conditionoccurs, a phase reversal in the sense winding is detected by a phasedetector and a pulse is applied to a relay drive circuit. The relaydrive circuit then activates the set coil of a relay to open the circuitbetween the source and the load. Means are provided for resetting therelay from an external source such as via telemetry from a groundstationrln addition means are provided for remotely opening the circuitbetween the source and the load.

it will be appreciated by those skilled in the art and others that onlya preferred embodiment of the invention has been described and thatother embodiments fall within the scope of the invention.

Iclaim:

1. An electronically resettable fuse comprising:

a saturable core current sensing transformer having one windingconnected so as to detect the current flow in a DC current flow path;

oscillator connected to said current sensing transformer to drive it ina balanced manner causing saturation of a predetermined magnetic path atwaveform peak points;

a phase detector having one input connected to said oscillator and asecond input connected to a sense winding wound about said currentsensing transformer for detecting a phase reversal between said twoinputs and for generating an output when said phase reversal occurs; and

circuit opening means connected to said phase detector for opencircuiting said DC current flow path when said phase detector eneratesan output signal. 2. An electromcaly resettablefuse as claimed in claim1,

wherein said current sensing transformer include two outside legs and acenter leg, said outside legs including balance windings connected tosaid oscillator; said winding connected so as to detect a current flowin a DC current flow path and said sense winding being both wound aboutsaid center leg.

3. An electronically resettable fuse as claimed in claim 2, wherein saidopen circuit means comprises a relay having a set winding connected tothe output of said phase detector.

4. An electronically resettable fuse as claimed in claim 3, wherein saidrelay includes a reset coil adapted to receive an external reset signal.

5. An electronically resettable fuse as claimed in claim 4, wherein saidphase detector applies a bias current to said sense winding.

6. An electronically resettable fuse as claimed in claim 5, including arelay drive circuit connected between the output of said phase detectorand the set coil of said relay.

7. An electronically resettable fuse as claimed in claim 6, wherein saidrelay drive circuit includes time delay components. I

8. An electronically resettable fuse as claimed in claim 7, wherein saidoscillator is a complementary transformer oscillator.

1. An electronically resettable fuse comprising: a saturable corecurrent sensing transformer having one winding connected so as to detectthe current flow in a DC current flow path; oscillator connected to saidcurrent sensing transformer to drive it in a balanced manner causingsaturation of a predetermined magnetic path at waveform peak points; aphase detector having one input connected to said oscillator and asecond input connected to a sense winding wound about said currentsensing transformer for detecting a phase reversal between said twoinputs and for generating an output when said phase reversal occurs; andcircuit opening means connected to said phase detector for opencircuiting said DC current flow path when said phase detector generatesan output signal.
 2. An electronically resettable fuse as claimed inclaim 1, wherein said current sensing transformer include two outsidelegs and a center leg, said outside legs including balance windingsconnected to said oscillator; said winding connected so as to detect acurrent flow in a DC current flow path and said sense winding being bothwound about said center leg.
 3. An electronically resettable fuse asclaimed in claim 2, wherein said open circuit means comprises a relayhaving a set winding connected to the output of said phase detector. 4.An electronically resettable fuse as claimed in claim 3, wherein saidrelay includes a reset coil adapted to receive an external reset signal.5. An electronically resettable fuse as claimed in claim 4, wherein saidphase detector applies a bias current to said sense winding.
 6. Anelectronically resettable fuse as claimed in claim 5, including a relaydrive circuit connected between the output of said phase detector andthe set coil of said relay.
 7. An electronically resettable fuse asclaimed in claim 6, wherein said relay drive circuit includes time delaycomponents.
 8. An electronically resettable fuse as claimed in claim 7,wherein said oscillator is a complementary transformer oscillator.